1. Field of the Invention
The present invention relates to a semiconductor laser having a mesa structure and capable of realizing stable traverse mode control, and a method for producing such a semiconductor laser.
2. Description of the Related Art
In a semiconductor laser structure, a buried type hetero structure where the periphery of an active layer is filled with a semiconductor material having a large energy gap and a small refractive index has been widely used. The semiconductor laser having such a buried type hetero structure has a low lasing threshold current value and excellent characteristics such as a stable transverse lasing mode so as to be utilized as a light source for optical fiber communication or optical information processing.
An exemplary structure of a conventional semiconductor laser having the buried type hetero structure is shown in FIG. 9.
In the conventional semiconductor laser 50 in FIG. 9, a mesa stripe 20 including an n-InP cladding layer 2, an InGaAsP active layer 3 and a p-InP cladding layer 4 is formed on the surface of an n-InP substrate 1 by etching. Thereafter, a p-InP current blocking layer 5 and an n-InP current blocking layer 6 are grown on the sides of the mesa stripe 20 by a liquid phase epitaxial growth method or vapor phase epitaxial growth method. Furthermore, a p-InP buried layer 7 and a p-InGaAsP contact layer 8 are grown so as to cover the mesa stripe 20 and the current blocking layers 5 and 6, thus forming the buried type hetero structure.
Next, after the entire surface of the thus formed buried type hetero structure is covered with a silicon oxide film 9, the silicon oxide film 9 in a region immediately above the active layer 3 is removed. Then, a p-type electrode 10 is formed in the region where the silicon oxide film 9 has been removed. Furthermore, a metal multilayered film 11 is deposited on the entire surface of the silicon oxide film 9 and the p-type electrode 10. Finally, an n-type electrode 12 is formed on the reverse face of the n-InP substrate 1.
When the thus produced semiconductor laser 50 is actually used, in order to improve heat release characteristics, as shown in FIG. 10, the semiconductor laser 50 is provided in a heat sink 13 with a soldering material 14 in a junction-down direction.
However, in the conventional semiconductor laser 50 having the above-mentioned structure, the silicon oxide film 9 (thermal conductivity: about 0.08 W/cm.multidot.K) surrounding the p-type electrode 10 and the underlying contact layer 8 formed of InGaAsP or InGaAs (thermal conductivity: about 0.05 W/cm.multidot.K) have lower thermal conductivities than InP (thermal conductivity: about 0.6 W/cm.multidot.K) forming the cladding layer 4. As a result, the layers 8 and 9 function as a barrier against thermal conduction so that heat generated between the p-type electrode 10 and the active layer 3 cannot be efficiently released to the heat sink 13.
Such a phenomenon in the heat release characteristic does not cause a serious problem during a low bias driving state with a driving current of, for example, about 100 mA. However, the phenomenon causes light output to be significantly saturated during a high bias driving state with a driving current of, for example, about 500 mA, thus adversely affecting the operating characteristics of the semiconductor laser.